From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [Qemu-devel] KVM call agenda for October 25 Date: Tue, 25 Oct 2011 09:06:20 -0500 Message-ID: <4EA6C25C.8000502@codemonkey.ws> References: <4EA6ACFE.6090109@redhat.com> <4EA6B41B.3000903@codemonkey.ws> <4EA6C00B.3030701@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Paolo Bonzini , qemu-devel@nongnu.org, kvm@vger.kernel.org To: Kevin Wolf Return-path: Received: from mail-yw0-f46.google.com ([209.85.213.46]:54310 "EHLO mail-yw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933537Ab1JYOGX (ORCPT ); Tue, 25 Oct 2011 10:06:23 -0400 Received: by ywm3 with SMTP id 3so513695ywm.19 for ; Tue, 25 Oct 2011 07:06:23 -0700 (PDT) In-Reply-To: <4EA6C00B.3030701@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 10/25/2011 08:56 AM, Kevin Wolf wrote: > Am 25.10.2011 15:05, schrieb Anthony Liguori: >> On 10/25/2011 07:35 AM, Kevin Wolf wrote: >>> Am 24.10.2011 13:35, schrieb Paolo Bonzini: >>>> On 10/24/2011 01:04 PM, Juan Quintela wrote: >>>>> >>>>> Hi >>>>> >>>>> Please send in any agenda items you are interested in covering. >>>> >>>> - What's left to merge for 1.0. >>> >>> I would still like to cache the default cache mode (probably to >>> cache=writeback). We don't allow guests to toggle WCE yet which Anthony >>> would have liked to see before doing the change. Is it a strict requirement? >> >> I don't see a way around it. If the default mode is cache=writeback, then we're >> open to data corruption in any guest where barrier=0. With guest togglable WCE, >> it ends up being a guest configuration issue so we can more or less defer >> responsibility. > > So do you think that offering a WCE inside the guest would be a real > solution or just a way to have an excuse? No, it offers a mechanism to "fix mistakes" at run-time verses at start up time. It also means that you can make template images that understand that they don't support barriers and change the WCE setting appropriately. > Christoph said that OSes don't usually change this by themselves, it > would need an administrator manually changing the setting. But if we > require that, we can just as well require that the administrator set > cache=writethrough on the qemu command line. The administrator of the guest != the administrator of the host. >> Do you think it's a good idea to change the default mode w/o guest WCE toggle >> support? What's your view about older guests if we change the default mode? >> What's your main motivation for wanting to change the default mode? > > Because people are constantly complaining about the awful > (cache=writethrough) performance they get before they are told they > should use a different cache option. And they are right. The > out-of-the-box experience with qemu's block performance really sucks. With qcow2 you mean, right? >> I'd be much more open to changing the default mode to cache=none FWIW since the >> risk of data loss there is much, much lower. > > I think people said that they'd rather not have cache=none as default > because O_DIRECT doesn't work everywhere. Where doesn't it work these days? I know it doesn't work on tmpfs. I know it works on ext[234], btrfs, nfs. Regards, Anthony Liguori > Kevin >